In recent years color televisions featuring liquid crystal (hereinafter referred to as LC) display screens have become practical. LC-based color panels consist of a quartz substrate having a matrix of thin film transistors (TFT) made from polycrystalline silicon, and an LC display substrate which is constructed by laminating a transparent filter glass substrate on top of the quartz substrate and sealing liquid crystal in the small gap between the quartz substrate and the glass plate. The functional characteristic of the LC-display is a twisted nematic mode. For imaging applications, the active-matrix driven display circuit arrangement is advantageous because of its adaptability to large area devices and to a high density of pixels and other circuit components. Practical applications have been implemented starting with relatively small display devices.
Such active-matrix driven LC panels are produced according to the following procedure. A transparent glass plate is placed, together with an intervening spacer, on top of a completed LC display substrate, and the small gap is filled with liquid crystal and sealed. LC display substrates usually contain as many as 250,000-500,000 pixels and more recent ones contain over 1,000,000 pixels.
Thin film processing steps are carried out in clean rooms to prevent small dust particles from causing problems in fabrication. However, as circuit density increases, even the minute amount of micron-sized dust particles, present naturally in the processing environment, are potential sources of open or short circuit defects for these micron size pixels and lines. The present standards allow up to ten such display defects per substrate, and those panels containing beyond this number are rejected. In other words, this number is the lowest defect level achievable by the present technology. As the panel becomes larger, the number of defects increases correspondingly, and the number of rejects increases accordingly. This is one reason for the high cost of large screen LC display devices, but the problems are further compounded by the difficulties associated with testing of such a large number of pixels.
The methods of testing pixels in LC display devices include the probing method, but this technique is inappropriate for such a large number of test objects which would require a large amount of costly testing time and effort. For this reason, the substrates are not tested during processing, but each substrate is visually evaluated by operating the display after it has been finish assembled into a display panel. At this late stage of manufacturing, even if imaging defects are discovered, the defective panels cannot be reprocessed. They are treated as rejects, and constitute a major reason for the poor yield of LC display devices. Further, visual inspection can only be qualitative and important quantitative information which could lead to process improvement is lacking.